Field Of the Invention: The present invention relates to respiratory flow measurement, and specifically to improving the performance of differential pressure flowmeters through enhanced signal processing.
State of the Art: Respiratory flow measurement during the administration of anesthesia and in intensive care environments provides valuable information for assessment of pulmonary function and breathing circuit integrity. Many different technologies have been applied to create a flowmeter that meets the requirements of the critical care environment. Among the flow measurement approaches which have been employed are:
1) Differential Pressure--measuring the pressure drop or differential across a resistance to flow. PA0 2) Spinning Vane--counting the revolutions of a vane placed in the flow path. PA0 3) Hot Wire Anemometer--measuring the cooling of a heated wire due to airflow passing around the wire. PA0 4) Ultrasonic Doppler--measuring the frequency shift of an ultrasonic beam as it passes through the flowing gas. PA0 5) Vortex Shedding--counting the number of vortices that are shed as the gas flows past a strut placed in the flow stream. PA0 6) Time of Flight--measuring the arrival time of an impulse of sound or heat created upstream to a sensor placed downstream.
Each of the foregoing approaches has various advantages and disadvantages, and an excellent discussion of most of these aforementioned devices may be found in W. J. Sullivan; G. M. Peters; P. L. Enright, M. D.; "Pneumotachographs: Theory and Clinical Application," Respiratory Care, July 1984, Vol. 29-7, pp. 736-49, and in C. Rader, Pneumotachography, a report for the Perkin-Elmer Corporation presented at the California Society of Cardiopulmonary Technologists Conference, October 1982.
At the present time, the most commonly employed device for respiratory flow measurement is the differential pressure flowmeter. Because the relationship between flow and the pressure drop across a restriction or other resistance to flow is dependent upon the design of the resistance, many different resistance configurations have been proposed. The goal of all of these configurations is to achieve a linear relationship between flow and pressure differential. It should be noted at this point that the terms "resistance" and "restriction" as applied herein to the physical configuration which produces a pressure drop or differential for use as a flowmeter input signal may be used interchangeably.
In theory, a differential pressure sensor across any resistance to flow constitutes a flowmeter. However, most flow resistances yield a differential pressure signal that is proportional to the square of the flow through the resistance. This "pressure squared" effect leads to a signal that is very small at low flows and is large at high flow rates. This phenomenon results in flowmeters that are either inaccurate at low flows or are incapable of measurements at high flows.
To accurately measure the pressure drop produced from very low flow through a restriction, the signal from the differential pressure transducer sensing the pressure must be amplified greatly to allow accurate reading. However, since the circuitry will not support signal voltages that are greater than its supply voltage, large voltage signals will be clipped off. Thus, the measurement of high flow rates will be limited by the gain of the amplifier circuitry. This limitation is compounded by the pressure squared effect. Traditionally, an optimal single, fixed gain is selected that will allow an acceptable dynamic range of operation for the flowmeter.
To some degree, the pressure squared effect can be compensated for in software of a processing unit associated with the pressure sensor of the flowmeter. If the pressure signal is digitized, the microprocessor can compensate for any non-linear relationship between differential pressure and flow. However, this compensation is limited by the resolution with which the signal is digitized. Currently, high resolution analog-to-digital converters are cost-prohibitive for use with differential pressure flowmeters. With limited analog-to-digital converter resolution, a one-bit change in the digitized pressure level may correspond to large flow differences at low flows, leading to poor accuracy at low flow.
In some prior art differential pressure flowmeters (commonly termed pneumotachs) the flow restriction has been designed to create a linear relationship between flow and differential pressure. Such designs include the Fleisch pneumotach in which the restriction is comprised of many small tubes or a fine screen, ensuring laminar flow and a linear response to flow. Another physical configuration is a flow restriction having an orifice variable in relation to the flow. This arrangement has the effect of creating a high resistance at low flows and a low resistance at high flows. Among other disadvantages, the Fleisch pneumotach susceptible to performance impairment from moisture and mucous, and the variable orifice flowmeter is subject to material fatigue and manufacturing variabilities.
One problem with pressure sensors employed in differential pressure pneumotachs is baseline drift. One technique, known as auto-referencing is used to compensate for baseline drift. The topic is discussed generally and with respect to flow measurement in "Basics of Auto-Referencing, " application note SSAN-2, SENSYM Catalog, 1991. In differential pressure pneumotach auto-referencing, valves are used to create a direct pneumatic connection between the two pressure ports of the differential pressure transducer. This shunt or bypass connection corresponds to zero flow through the flow resistance. The pressure measurement at zero flow is sorted and subtracted from future measurements to compensate for baseline drift. This subtraction is normally done digitally in a computer program, but might also be performed using an analog circuit. However, the digital subtraction technique currently requires either a linear flowmeter or high resolution analog to digital conversion of the non-linear pressure signal.